1. Field of the Invention
The present invention relates to a power-supply device where, independently of a power L/C smoothing filter, a signal is caused to pass through a CR smoothing filter and is then fed back so that the control loop will be stabilized.
2. Description of the Related Art
A prior art on the loop stabilizing method for a power-supply device has bene described in “Low-Voltage On-Board DC/DC Modules for Next Generations of Data Processing Circuits”, Zhang et al., IEEE Tran. on Power Elect. Vol. 11, No. 2, March 1996. In the power-supply device according to the prior art, a signal is fed back to an error amplifier from a power LC smoothing filter. Then, the error amplifier compensates the phase, thereby implementing the stabilization of the control loop. In this prior art, an aluminum electrolytic capacitor is used as the power LC smoothing filter.
U.S. Pat. No. 5,877,611 discloses a power supply system in which an output of a CR smoothing filter connected across an inductor of an output LC smoothing filter is fed back to an error amplifier having a low input impedance. In the U.S. patent prior art, voltage and current signals of a power supply output are extracted using the CR smoothing filter, so that the resistance value of the CR smoothing filter must be set to be small. The component constants of the CR smoothing filter are a capacitance of 0.47 μF and a resistance of 100 Ω. Accordingly, the Cr smoothing filter having such constants cannot be formed on chip in a power supply IC and must be formed externally of the IC chip, resulting in a problem that the power supply device cannot be made in small size totally.